The present invention concerns systems for monitoring the pressure of tires of motor vehicles, comprising, on each wheel, a pressure sensor and a transmitter for transmitting the pressure to a monitoring element.
In these systems, which are referred to in the art as tire pressure monitoring systems (TPMS), the signals transmitted by the pressure sensors include data which allow them to be identified. If the wheels of a vehicle were never changed, a simple initial entry, at the factory, of the allocation between signals and wheels would allow a determination of the location of the sensors with respect to the vehicle's chassis. This determination is necessary in order to locate defective tires.
Since the wheels of a vehicle are, however, occasionally put in different positions and rotated, the problem still exists of locating the wheel on which the sensor is mounted.
Some solutions have already been proposed. In particular, the Applicant describes in French Patent Application FR 0 116 368 a system for monitoring tire pressure in which rotation speeds calculated on the basis of data supplied by the sensors are compared to those determined by fixed wheel rotation speed sensors whose location is known. The latter are, in particular, antilock braking system (ABS) sensors. The ABS uses a separate sensor for each wheel that provides its angular position in the form of pulses. The latter correspond to distances traveled by the wheel (on the order of a few centimeters).
It is known that the wheels of a vehicle do not all rotate at the same speed. A difference in the pressure of the tires, for example, or even in the amount of wear on the tread, results in a difference in the wheels' diameter and thus in their rotation speed. Vehicle dynamics, load distribution, and temperature are also factors influencing the rotation speeds of wheels with respect to one another.
These deviations are thus utilized in order to make this determination. To that end, a calculation is made of the difference between the speed of each fixed sensor and that calculated on the basis of data supplied by the pressure sensors. It is thus possible to associate with each of the fixed sensors that pressure sensor having the lowest calculated speed deviation. This system is advantageous in that it uses existing means, i.e. at no extra cost. It has also proven to be reliable, given the quality of the fixed sensors of the antilock braking system.
It is not uncommon, however, for the measured speeds to be very similar to one another. Calculation convergence is then slow, and the system's reaction time is relatively long (as long as 60 minutes). The pressure sensors are powered by batteries located inside the tires. It is desirable to limit their energy consumption as much as possible in order to increase the product's service life. A limitation of the sensors' transmission time is therefore advantageous.
The Applicant has thus established as its object that of improving the means of locating wheels in a tire pressure monitoring system using a means with which the transmission time of the sensors can be reduced.